In a 3GPP system, a “soft handover” may occur if a mobile terminal or UE (User Equipment) is handed over to two base stations or Node Bs (where the UE can be connected to two or more Node Bs at the same time). The attachment to the new Node B is considered a “hard handover” if the UE lies in Cell-DCH (Dedicated CHannel) state or a “cell reselection” if the UE lies in Cell-FACH (Forward Access CHannel) state as described in the 3GPP Technical Specification 3GPP TS 25.331, available from the 3GPP website at www.3gpp.org.
In 3GPP, it is known that there are two methods to accommodate the handover of a UE between RNCs (Radio Network Controllers). In a first known method the UE's protocols (RRC—Radio Resource Control—and RANAP—Radio Access Network Application Part) are left untouched, but the signalling and user plane traffic is forwarded over the lur interface to the new RNC that now parents the Node B communicating to the UE. This new RNC is called the Drift RNC (DRNC), whilst the RNC with the RRC entity is called the Serving RNC (SRNC). These terms are particular to the UE.
The advantages of this method are:                it is transparent to the SGSN (Serving GPRS Support Node)        there is no need to move the UE's protocols and contexts from the SRNC, and since network signalling is only required between two peer (RNC) elements handover should be rapid.        
However, using a DRNC in this way does introduce some difficulties:                the radio resources of the Node B are managed by the radio resource management (RRM) within the DRNC, but the measurements performed by the UE to support this terminate at the SRNC, and must then be forwarded over the lur interface        The RRM is unable to manage the loading or resource consumption of traffic on dedicated channels that are scheduled at the UE and SRNC        Flow control of user plane traffic on common channels is required on the lur interface        The extended path of the RRC protocol increases signalling latency between the UE and the radio network        Traffic loading on the lur interface increases        A fully compliant lur interface is required to carry user and control plane data        
A second known method, alternative to using a DRNC in 3GPP, is to perform a SRNS (Serving Radio Network System) Relocation. In this procedure the UE's protocols within the radio network are moved to the new RNC. The lu connection used is also switched from the SGSN to the new RNC, for user and control planes. The new RNC is known as the Target RNC during the relocation. To do this, extensive signalling takes place between the old and new RNCs and the SGSN (using RANAP) and also (cell reselection only) between the RNCs themselves over the lur interface (using RNSAP—Radio Network Subsystem Application Part—protocol).
The advantages and disdavantages of such SRNS Relocation are the opposite of those of using the DRNS. Clearly, to perform this method the SGSN must support relocation. The rôles it plays are:                Receiving RANAP messages from one RNC, recycling each message, and forwarding it to the other RNC        In doing this the SGSN provides information about the radio access bearers existing for the UE (RANAP Relocation Request message) to the target RNC        Switching over the lu connection        
Other known handover techniques include that used in GSM (Global System for Mobile communications). In GSM, when a UE hands over to a cell under the jurisdiction of a new MSC (Mobile Switch Centre), the call is routed through the old MSC, the anchor MSC. Since in GSM a BSC (Base Station Controller)—BSC interface does not exist, it is not possible to use an anchor BSC. The anchor MSC allows simple billing as the UE moves.
3GPP SRNS Relocation as described above, and variants thereof, are known from a variety of patent publications:                US2003036387 assumes soft handover and is not applicable        US2003003919 discloses 3GPP SRNS Relocation, with an emphasis on the use of NBAP (Node B Application Part) and RNSAP messages to establish a new transport bearer between target RNC and Node B        WO0176282 discloses a technique in which the SRNC tells the Target RNC which transport channels it should use for the UE        EP1337125 addresses the transfer of ciphering info, source to target        US2001046218 considers synchronised handover (framing alignment, FDD mode only)        US2003007490 discloses lossless SRNS Relocation        WO002065796 discloses the control and user planes being split on the lu, with only the user plane being relocated.        
However, the disclosed systems are suboptimal in many situations and tend to result in a complex or inflexible system, to be impractical for many applications and/or to result in suboptimal performance and excessive signalling.
Hence, an improved system for radio network relocation of a mobile station would be advantageous.